Abstract

Background

In wheat (Triticum aestivum L), the flag leaf has been thought of as the main source of assimilates for grain
growth, whereas the peduncle has commonly been thought of as a transporting organ.
The photosynthetic characteristics of the exposed peduncle have therefore been neglected.
In this study, we investigated the anatomical traits of the exposed peduncle during
wheat grain ontogenesis, and we compared the exposed peduncle to the flag leaf with
respect to chloroplast ultrastructure, photosystem II (PSII) quantum yield, and phosphoenolpyruvate carboxylase (PEPCase; EC 4.1.1.31) activity.

Results

Transmission electron microscope observations showed well-developed chloroplasts with
numerous granum stacks at grain-filling stages 1, 2 and 3 in both the flag leaf and
the exposed peduncle. In the exposed peduncle, the membranes constituting the thylakoids
were very distinct and plentiful, but in the flag leaf, there was a sharp breakdown
at stage 4 and complete disintegration of the thylakoid membranes at stage 5. PSII
quantum yield assays revealed that the photosynthetic efficiency remained constant
at stages 1, 2 and 3 and then declined in both organs. However, the decline occurred
more dramatically in the flag leaf than in the exposed peduncle. An enzyme assay showed
that at stages 1 and 2 the PEPCase activity was lower in the exposed peduncle than
in the flag leaf; but at stages 3, 4 and 5 the value was higher in the exposed peduncle,
with a particularly significant difference observed at stage 5. Subjecting the exposed
part of the peduncle to darkness following anthesis reduced the rate of grain growth.

Conclusion

Our results suggest that the exposed peduncle is a photosynthetically active organ
that produces photosynthates and thereby makes a crucial contribution to grain growth,
particularly during the late stages of grain-filling.